|Biological agent type
Thistle stem hover fly.
Plant species attacked
bull thistle (Cirsium vulgare)
Italian thistle (Carduus pycnocephalus)
musk thistle (Carduus nutans)
slenderflower thistle (Carduus tenuiflorus)
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Images by Eric Coombs, Oregon Department of Agriculture.
If images are downloaded and used from the ODA web site please be sure to credit the photographer.
Site of attack
In slenderflower and Italian thistles larvae attack the early buds. The larvae can cause bud die-offs as they mine into the lower stem. Larvae then tunnel down the stem to the root crowns.
Mode of attack
The larvae mine the stem and root crown and leaf petioles of some thistles.
Collection and release information
This insect is widespread in Douglas County. Consult with ODA for availability for your area.
History in Oregon
In 1991, the crown gall fly Cheilosia corydon was released in Malheur County, near Vale. The fly was re-released again in 1993 and failed to establish. This agent was released in 1991 in Douglas County, near Dixonville. The fly was re-released again in 1993 and is now established in Douglas County on slenderflower and Italian thistles. In 1999, the fly was discovered for the first time in the U.S., near Dixonville, and had spread nearly 5 miles from the release site. The fly is now widely established throughout Douglas County where the host occurs. Larvae were found in over 50% of the stems greater than 10 mm in diameter. The narrow stemmed Italian thistle is infrequently used as a host. The fly is primarily known as an agent for musk thistle. Damage similar to that caused by Cheilosia has been observed in several native thistles species (Cirsium edule group), thus caution should be used when considering introduction of this fly into new areas.
Release status in Oregon
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